Interest Rate Calculator
Reverse-engineer the annual interest rate (APR) of any loan from its amount, monthly payment, and term.
Example
Borrow $20,000, repay $400/month over 5 years (60 payments). Total repaid is $24,000, so $4,000 is interest. Solving the amortization equation by bisection gives a monthly rate of about 0.618%, an APR of roughly 7.42%.
How it works
Enter the loan amount, your monthly payment, and the term; the tool solves the amortization equation by bisection to find the monthly rate, then reports the annualized APR. Total interest and total repaid update instantly.
Good to know
This Interest Rate Calculator works backward from the numbers you already know. Instead of asking for a rate and telling you the payment, it takes a loan amount, a fixed monthly payment, and a term, then solves for the annual interest rate (APR) that makes those figures line up under standard fixed-rate amortization. It is useful when a quote, statement, or older paperwork shows the balance and payment but not the rate, or when you want to sanity-check a deal someone has offered you.
Reach for it in situations like comparing a dealer or store financing offer against a bank loan, decoding the implied rate on a car loan or personal loan, or working out roughly what you are paying on an existing fixed installment debt. Because the math runs entirely in your browser, you can plug in real numbers without sending anything to a server.
To read the result, start with the headline APR, then use the monthly rate (APR divided by 12), total interest, and total repaid to see the whole cost. The principal-versus-interest bars show what share of your payments is genuine cost versus repaying what you borrowed. A higher interest bar relative to principal signals an expensive loan, a long term, or both.
One caveat worth keeping in mind: this figure is derived only from amount, payment, and term, so it reflects the pure amortization rate, not necessarily the lender's official APR, which may also bundle in fees, points, or insurance. If your total of all payments is less than the amount borrowed, no positive rate can exist and the tool will say so. Treat the output as a comparison and verification estimate rather than a contractual number.
Frequently asked questions
Why does my lender's quoted APR differ from this result?
This tool derives the rate purely from amount, payment, and term using standard amortization. A lender's APR may also fold in origination fees, points, or insurance, and may use a different compounding convention, which shifts the figure.
What if it says no positive rate exists?
That happens when your total of all payments is less than the loan amount (monthly payment x number of months < principal). The loan can't be repaid under those terms, so increase the payment or lengthen the term.
Is my data uploaded anywhere?
No — this calculator runs entirely in your browser; nothing is uploaded.
Is this financial advice?
No. These are educational estimates — consult a qualified financial professional before making decisions.
People also ask
How do you calculate the interest rate when you only know the loan amount, payment, and term?
There is no simple closed-form formula to solve the amortization equation for the rate, so calculators find it numerically. This tool uses bisection: it repeatedly tests candidate monthly rates, checks the resulting payment, and narrows the range until the calculated payment matches your actual payment, then annualizes that monthly rate into an APR.
Is APR the same as the monthly interest rate?
No. The monthly rate is the periodic rate charged each month, and this tool multiplies it by 12 to get the APR. For example, a 0.618% monthly rate corresponds to roughly a 7.42% APR.
Why is total interest just total payments minus the loan amount?
On a fully amortizing fixed loan, everything you pay beyond the original principal is interest. So if you borrow 20,000 and repay 24,000 in total, the 4,000 difference is the interest cost over the life of the loan.
Does a lower monthly payment mean a lower interest rate?
Not necessarily. A lower monthly payment often comes from stretching the term over more months, which can keep payments small while actually increasing total interest paid and sometimes implying a higher rate. The total interest and total repaid figures show the real cost behind the payment size.
What is the difference between interest rate and APR on a real loan?
The interest rate is the cost of borrowing the principal, while a lender's APR is meant to reflect interest plus certain fees such as origination charges or points, expressed as a yearly percentage. This calculator reports the implied rate from your payment numbers, which may be lower than a lender's stated APR if fees are involved.
Can this calculator handle loans with a balloon payment or variable rate?
No. It assumes a standard fixed-rate, fully amortizing loan with equal monthly payments. Loans with balloon payments, changing rates, or irregular schedules will not match this model accurately.
What term unit should I enter, years or months?
You can use either. Selecting Years multiplies your term value by 12 internally, while Months uses the number directly, so a 5 with Years and a 60 with Months produce the same result.
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